The present invention relates to a process for coding binary signals for numbering images or pictures stored on a recording medium, particularly a video disk and to the device for reading the thus coded signals.
It is known to record on a medium television signals which conventionally comprise a video component and a sound component. The recorded television signals are obtained from the analysis of images in one or more fields, each image being for example an uneven number of lines distributed over two fields, 625 lines in Europe. European television systems operate with a frequency of 50 fields per second. Analysis of the images takes place in accordance with two interlaced fields, which gives 25 images per second. Other systems which are used, particularly in the USA, such as NTSC operate with a frequency of 60 fields per second.
Operational signals such as synchronizing signals are also recorded during the line and image return intervals. The field return intervals can also incorporate binary image numbering signals. During the reading process, this information is used on the one hand for finding a predetermined image and on the other to obtain certain special effects, such as accelerated or decelerated image stopping.
On a video disk, the images are recorded on a spiral track or in concentric circles. In the case of a frequency of 50 fields per second, a complete image is recorded on each turn of the track, i.e. one complete image or two fields can be read for each revolution of the video disk. Stopping on the image can take place merely by jumping from one track to the previous track after one revolution of the video disk. However, this is not possible for a frequency of 60 fields per second, because each image must have 3 fields, corresponding to one and a half revolutions of the video disk.
French Patent Application No. 77 000 79, filed on Jan. 4, 1977 and published under No. 2,376,486 proposes a device making it possible to bring about a correct stopping on the image, no matter what process is used for recording these images on the disk, i.e. SECAM or PAL, 50 Hz, 625 lines or NTSC, 60 Hz, 525 lines. A binary-coded number is associated with each field and the numbers associated with the fields of one and the same image are identical. Thus, only the smallest weight bit varies from one image to the next. The jump pulse generating system described in the aforementioned patent application uses this variation to establish whether the field which is read does or does not come from the same images as the previously read field. Thus, for this device to operate correctly, it is necessary to ensure that no reading error or any type of deterioration makes it impossible to restore the image number asigned to one of the fields and even possibly to 2 or 3 fields constituting the image on which a stop is to be made. It is merely necessary for the error to affect the lowest weight bit of the image number to prejudice the operation of the device.